Using membrane contactors for fruit juice concentration

Osmotic evaporation is a membrane process that has fascinating advantages over conventional processes for the production of fruit juice concentrates since it has the ability to concentrate solutes to very high levels with minimal thermal or mechanical damage. Our purpose was to compare and analyse the results obtained in the concentration of a sucrose solution used as model juice. Two different membrane contactors with different specific areas and number and dimensions of fibres/tubes were used. The hydrodynamic conditions that maximize the water flux were previously determined by using a series resistance model. For a determined range of Reynolds number, the boundary layer mass transfer resistances can be neglected and there is only the contribution of the membrane resistance. The concentration of a sucrose solution from 12° Brix to 60º Brix, used as a model fruit juice, was carried out. The water flux decreased due to the progressive reduction of the driving force. The overall mass transfer coefficient, Kp, remained constant throughout the experiment, except for one of the contactors used where there was a reduction due to the concentration polarization caused by the increase of the sucrose solution’s viscosity.

The concentration process of a sucrose solution by osmotic evaporation using two membrane contactors was studied. The hydrodynamic conditions that maximise the water flux were previously determined, i.e., the range of Reynolds number to be used in order that the boundary layer mass transfer resistances can be neglected. The sucrose solution used as a model fruit juice was concentrated from 12º Brix to 60 ºBrix. This final concentration was achieved in a lower operation time using contactor I due to its higher membrane area, 0.16 m2, and membrane mass transfer coefficient, (2.53±0.07)×10!10 ms!1 Pa!1, than that of contactor II (0.036 m2 and (2.33± 0.14)×10!11 ms!1 Pa!1, respectively). The water flux decreased due to the progressive reduction of the driving force, but the overall mass transfer coefficient, Kp, remained constant throughout the experiment. However, for contactor I there is a reduction in the last hour due to the concentration polarization caused by the higher fluxes and by the increase of the sucrose solution’s viscosity.